1. Field of the Invention
The invention relates generally to optical phase shifting and in particular to apparatus and methods for achieving low cost and yet relatively rapid discrete phase shifting in optical interferometers.
2. Description of Related Art
Optical coherence tomography (OCT) is a technology that is based on low coherence optical interferometry to scan a sample in both the depth and transverse directions to generate a two or three dimensional image of the sample (Huang, D. et al. (1991). “Optical coherence tomography.” Science 254 (5035): 1178-81; and Fujimoto, J. G. “Optical coherence tomography for ultrahigh resolution in vivo imaging.” Nat Biotechnol 21(11): 1361-7, (2003)). This technology was first implemented in the time domain, in which the relative optical path length difference of the two interferometer arms is scanned, typically by moving a reference mirror mechanically as a function of time (U.S. Pat. Nos. 5,459,570, 5,321,501). This approach limits image acquisition speed.
It was later demonstrated that spectral domain OCT has significant advantages in speed. In spectral domain OCT, the optical path length difference between the sample and reference arm is not mechanically scanned but rather, the interferometrically combined beam is sent to a spectrometer in which different wavelength components are dispersed onto different photodetectors to form a spatially oscillating interference fringe (Smith, L. M. and C. C. Dobson (1989). “Absolute displacement measurements using modulation of the spectrum of white light in a Michelson interferometer.” Applied Optics 28(15): 3339-3342). A Fourier transform of the spatially oscillating intensity distribution can provide information on the reflectance distribution as a function of depth within the sample. As there is no mechanical depth scanning, acquisition of reflected light which covers a full depth range within the sample can be achieved simultaneously, and consequently, the speed of obtaining a full depth reflection image is substantially increased as compared to time domain OCT (Wojtkowski, M., et al. (2003). “Real-time in vivo imaging by high-speed spectral optical coherence tomography.” Optics Letters 28(19): 1745-1747; Leitgeb, R. A., et al. (2003). “Phase-shifting algorithm to achieve high-speed long-depth-range probing by frequency-domain optical coherence tomography.” Optics Letters 28(22): 2201-2203). In addition, as the light reflected from the full depth range within the sample is fully dispersed over many photodetectors, the shot noise for each photodetector is substantially reduced as compared to the time domain OCT case, and hence the signal to noise ratio can also be substantially increased (Leitgeb, R. A., et al. (2003). “Performance of Fourier domain vs. time domain optical coherence tomography.” Optics Express 11(8): 889-894; De-Boer, J. F., et al. (2003). “Improved signal-to-noise ratio in spectral-domain compared with time-domain optical coherence tomography.” Optics Letters 28(21): 2067-2069).
However, a direct Fourier transform of the interferogram is not sufficient to provide the information on the complex reflectance distribution within the sample. Such a direct Fourier transform contains both the autocorrelation and the cross-correlation interference terms and it does not reveal the phase between the sample and the reference reflector. Note that the superimposed intensity interferogram from a positive optical path length difference will be the same as that from a negative optical path length. To solve this problem, Fercher et al. used Fourier transformation of the complex spectral distribution, that is, a Fourier transformation of both the amplitude and the phase of the light beam (Fercher, A. F. et al. (1995). “Measurement of intraocular distances by backscattering spectral interferometry.” Optics Communications 117(1-2): 43-48; and U.S. Pat. No. 6,377,349). In this method, the reference beam is changed in phase by discrete steps to separate the superimposed interferograms of difference phases, and the amplitude and phase of the sample beam are then obtained from the complex spectral distribution of the interferogram. An associated benefit of such a discrete relative phase shifting technique is that the usable depth coverage range is also doubled (Wojtkowski, M., A. Kowalczyk, et al. (2002). “Full range complex spectral optical coherence tomography technique in eye imaging.” Optics Letters 27(16): 1415-1417; Leitgeb, R. A., et al. (2003) “Phase-shifting algorithm to achieve high-speed long-depth-range probing by frequency-domain optical coherence tomography.” Optics Letters 28(22): 2201-2203). In addition, the DC background and the contribution from autocorrelation interference terms can also be removed (Vakhtin, A. B., et al. (2003). “Differential spectral interferometry: an imaging technique for biomedical applications.” Optics Letters 28(15): 1332-1334).
However, all the prior art phase shifted SD-OCT system used a mirror attached to a piezoelectric ceramic stack or a mirror surfaced ceramic stack to achieve the desired discrete relative phase shift. This approach is slow due to the limited frequency response as well as the resonance of the ceramic stack. While there are already various types of optical phase shifters, modulators and optical path length delay lines as will be discussed shortly, the present invention discloses a novel way to achieve relatively high speed discrete optical phase shifting.
Optical phase shifting or modulation has been widely used in optical interferometry for various purposes, including optical wave front shaping, optical intensity modulation, and interferometric quadrature condition maintaining. In connection with interferometric optical fiber sensors, piezoelectric ceramic materials have been used to stretch a piece of fiber to achieve optical phase modulation. In free space based optical interferometers or laser cavities, mirrored piezoelectric ceramics have been used in reflection to achieve optical phase or path length modulation. Other electro or magneto-mechanically expandable or contractible materials such as piezoelectric polymers and magnetostrictive ceramics have also been used for similar purposes. However, these materials generally can only be operated up to a frequency of about several tens of kHz and they also have inherent resonance frequencies and hysteresis, and hence they cannot easily be operated to provide high speed stepped optical phase shifting. There are reports on fiber optic phase modulators that are made by coating thick lead zirconate titanate coaxially on the fiber which can achieve operating speeds within a range from hundreds of kilohertz to several megahertz. However, such a device is not commercially available yet and its cost is expected to be much higher than the present invention.
In optical fiber communications, electro-optic crystals and integrated optical waveguides are widely used for optical phase modulation. These optical phase modulators are based on the change in the refractive index of a crystal material in response to a change in the electrical or magnetic field or injected free-carriers applied to the material. They can be used to provide light intensity modulation using a Mach-Zehnder interferometer and the modulation speed is very high (up to tens of GHz). However, the cost of these devices is also high. In addition to the high cost, when a bulk crystal is used for optical phase modulation, the required voltage is also very high, whereas with integrated optical waveguide, light coupling into and out of the waveguide will most likely cause substantial light insertion loss as well as high cost associated with the packaging of the device.
Rotating devices have been used to vary the path length in time domain OCT systems. Examples include: a corner cube attached at an offset from a rotating shaft (U.S. Pat. Nos. 5,459,570, 5,321,501), a rotating helical mirror (U.S. Pat. No. 5,491,524), a rotating galvo mirror of a rapid scanning optical delay (RSOD) line (U.S. Pat. Nos. 6,111,645, 6,282,011), a rotary prism array (Lai, M. (2001). “Kilohertz Scanning Optical Delay Line Employing a Prism Array.” Applied Optics 40(34): 6334-6336.) and a rotary mirror array (Chen, N. G. and Q. Zhu (2002). “Rotary mirror array for high-speed optical coherence tomography.” Optics Letters 27(8): 607-609.). The purpose of these devices was to smoothly vary the path length of the reference arm relative to the sample arm of the time domain OCT system to vary the depth at which sampling occurs. The purpose of presently invented spinning disk is to shift the phase of the reference arm light relative to the sample arm in typically sub-wavelength increments over a range of approximately a wavelength to obtain quadrature information on the interference between the reference arm and the sample arm.